Solid electrolyte, method of preparing the same, and secondary battery including the same

ABSTRACT

A solid electrolyte including an inorganic lithium ion conductive film and a porous layer on a surface of the inorganic lithium ion conductive film, wherein the porous layer includes a first porous layer and a second porous layer, and the second porous layer is disposed between the inorganic lithium ion conductive film and the first porous layer, and wherein the first porous layer has a size greater which is than a pore size of the second porous layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2018-106504, filed on Sep. 6, 2018, in the KoreanIntellectual Property Office, and all the benefits therefrom under 35U.S.C. § 119, the content of which is incorporated herein in itsentirety by reference.

BACKGROUND 1. Field

The present disclosure relates to a solid electrolyte for a secondarybattery, a method of preparing the same, and a secondary batteryincluding the same.

2. Description of the Related Art

With the explosive growth of reusable energy storage devices applicableto electric vehicles and portable electronic devices, there is anincreasing need for a lithium secondary battery having high capacity andimproved stability. A lithium metal electrode, as a negative electrodefor a lithium secondary battery, has been investigated as an option toincrease charge storage capacity and provide a secondary battery havinga high voltage.

However, when an inorganic solid electrolyte is used as an electrolytein a lithium secondary battery including a lithium metal electrode, ashort circuit may occur due to lithium penetration into grain boundariesof the inorganic solid electrolyte, and interfacial resistance betweenthe lithium metal electrode and the solid electrolyte may increase.

There thus remains a need for an improved lithium secondary batterysolid electrolyte.

SUMMARY

Provided is a solid electrolyte and a method of preparing the same.

Provided is a secondary battery including the solid electrolyte.

Additional aspects will be set forth in part in the description whichfollows and, in part, will be apparent from the description, or may belearned by practice of the presented embodiments.

According to an aspect of an embodiment, a solid electrolyte includes aninorganic lithium ion conductive film; and a porous layer on a surfaceof the inorganic lithium ion conductive film, wherein the porous layercomprises a first porous layer and a second porous layer, and the secondporous layer is disposed between the inorganic lithium ion conductivefilm and the first porous layer, and wherein the first porous layer hasa pore size which is greater than a pore size of the second porouslayer.

According to an aspect of another embodiment, a secondary batteryincludes a positive electrode, a negative electrode, and the solidelectrolyte interposed between the positive electrode and the negativeelectrode.

According to an aspect of another embodiment, a method of preparing asolid electrolyte includes: a first acid treatment comprisingacid-treating an inorganic lithium ion conductive film with an acidhaving a concentration of greater than or equal to about 0.1 molar (M)and less than or equal to about 5 molar to provide a first acid-treatedproduct; and a first cleaning comprising cleaning the first acid-treatedproduct to provide a cleaned first acid-treated product to prepare thesolid electrolyte.

The inorganic lithium ion conductive film may be prepared by mixing aninorganic lithium ion conductor and a lithium compound to prepare amixture, and heat-treating the mixture.

The method further includes a second acid treatment includingacid-treating the cleaned first acid-treated product with an acid havinga concentration of more than 0.1 M and less than 0.5 M to provide asecond acid-treated product, and a second cleaning process includingcleaning the second acid-treated product.

According to an aspect of another embodiment, a method of preparing asolid electrolyte includes forming a multilayer film on a surface of aninorganic lithium ion conductor film, the forming including: coating afirst composition comprising a pore former and a lithium ion conductoron a surface of the inorganic lithium ion conductive film, and dryingthe first composition, coating a second composition comprising a poreformer and a lithium ion conductor on the dried first composition, anddrying the second composition to form the multilayer film; andheat-treating the multilayer film to prepare the solid electrolyte.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects will become apparent and more readilyappreciated from the following description of the embodiments, taken inconjunction with the accompanying drawings in which:

FIG. 1 is a schematic diagram illustrating an embodiment of a structureof a solid electrolyte;

FIG. 2 is a schematic diagram illustrating another embodiment of astructure of a solid electrolyte according;

FIG. 3 is a schematic diagram illustrating an embodiment of a structureof a secondary battery;

FIG. 4A is a scanning electron microscopic (SEM) image of across-section of a solid electrolyte prepared according to Example 1, inwhich the horizontal dashed lines delineate the layered structure;

FIG. 4B is an enlarged area of the encircled portion of the image ofFIG. 4A;

FIG. 4C is an SEM image of a cross-section of a solid electrolyteprepared according to Comparative Preparation Example 1-1, in which thedashed lines delineate the layered structure;

FIG. 5A is a graph of cumulative intrusion (milliliters per gram, mL/g)versus pore diameter (micrometers, μm), illustrating pore sizedistribution and mechanical strength (Ring on Ring test) of the solidelectrolyte prepared according to Examples 6 and 7;

FIG. 5B is a graph of incremental intrusion (mL/g) versus pore diameter(μm), illustrating pore size distribution and mechanical strength (Ringon Ring test) of the solid electrolyte prepared according to ComparativeExample 2;

FIG. 6A is a graphs of counts (arbitrary units) versus diffraction angle(degrees 2-theta) illustrating the results of X-ray diffraction (XRD)analysis of an LLZO Film prepared according to Preparation Example 1 anda solid electrolyte prepared according to Example 1, respectively;

FIG. 6B is an expanded portion of the encircled portion of FIG. 6A;

FIGS. 7A and 7B are graphs of imaginary impedance (−Z, ohm²) versus realimpedance (Z′, ohm²) illustrating the impedance characteristics oflithium secondary batteries prepared according to Example 8 andComparative Example 3, respectively;

FIGS. 8A to 8C are graphs of electrode potential (volts versus Li/Li⁺)versus capacity (milliampere-hours per square centimeter, mAh·cm⁻²),illustrating charge/discharge characteristics of lithium secondarybatteries prepared according to Example 8 and Comparative Examples 3 and4, respectively;

FIG. 9A is a graph of capacity (mAh·cm⁻²) and efficiency (percent, %)versus cycle number, illustrating the capacity and efficiency of thelithium secondary battery prepared according to Example 8;

FIG. 9B is a graph of electrode potential (volts versus Li/Li⁺) versuscapacity (mAh·cm⁻²), illustrating the electrode potential of a lithiumsecondary battery prepared according to Example 8;

FIG. 9C is a graph of capacity (mAh·cm⁻²) versus cycle number,illustrating the capacity of the lithium secondary battery preparedaccording to Comparative Example 8;

FIGS. 10A to 10C are graphs of intensity (arbitrary units, a.u.) versusbinding energy (electron volts, eV), illustrating the results of Li 1s,C 1s, and O 1s X-ray photoelectron spectroscopy (XPS) analysis of anLLZO film prepared according to Preparation Example 1, respectively;

FIGS. 10D to 10F are graphs of intensity (a.u.) versus binding energy(eV) illustrating the results of Li 1s, C 1s, and O 1s XPS analysis of asolid electrolyte prepared according to Example 1, respectively;

FIGS. 11A and 11B are graphs of imaginary impedance (−Z, ohm²) versusreal impedance (Z′, ohm²), illustrating the impedance results ofEvaluation Example 7, which is an aging test of the lithium secondarybatteries prepared according to Comparative Example 3 and Example 8,respectively;

FIG. 12A is a histogram illustrating the relative amount of protons forthe solid electrolytes of Example 1 and Preparation Example 1 at thesurface and within the solid electrolyte, illustrating the results ofLaser Induced Breakdown Spectroscopy (LIBS) analysis of a solidelectrolyte prepared according to Example 1 and an LLZO Film preparedaccording to Preparation Example 1;

FIG. 12B is a graph of relative amount of protons versus distance fromthe surface (micrometers, μm) illustrating the results of (LIBS)analysis of a solid electrolyte prepared according to Example 1 and anLLZO Film prepared according to Preparation Example 1;

FIGS. 13A and 13B are graphs of electrode potential (volts versusLi/Li⁺) versus capacity (mAh·cm⁻²) illustrating the electrode potentialof lithium secondary batteries prepared according to Examples 8 and 11with respect to capacity; and

FIGS. 14A and 14B are graphs of electrode potential (volts versusLi/Li⁺) versus capacity (mAh·cm⁻²), illustrating the electrode potentialof lithium secondary batteries prepared according to ComparativeExamples 6 and 7.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments, examples of whichare illustrated in the accompanying drawings, wherein like referencenumerals refer to like elements throughout. In this regard, the presentembodiments may have different forms and should not be construed asbeing limited to the descriptions set forth herein. Accordingly, theembodiments are merely described below, by referring to the figures, toexplain aspects.

It will be understood that when an element is referred to as being “on”another element, it can be directly on the other element or interveningelements may be present therebetween. In contrast, when an element isreferred to as being “directly on” another element, there are nointervening elements present.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a,” “an,” and “the” are intended to include the pluralforms, including “at least one,” unless the content clearly indicatesotherwise. “At least one” is not to be construed as limiting “a” or“an.” As used herein, the term “and/or” includes any and allcombinations of one or more of the associated listed items. Expressionssuch as “at least one of,” when preceding a list of elements, modify theentire list of elements and do not modify the individual elements of thelist.

As used herein, the term “and/or” includes any and all combinations ofone or more of the associated listed items. It will be furtherunderstood that the terms “comprises” and/or “comprising,” or “includes”and/or “including” when used in this specification, specify the presenceof stated features, regions, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, regions, integers, steps, operations, elements,components, and/or groups thereof.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper” and the like, may be used herein for ease of description todescribe one element or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the exemplary term “below” can encompass both anorientation of above and below. The device may be otherwise oriented(rotated 90 degrees or at other orientations) and the spatially relativedescriptors used herein interpreted accordingly.

“About” or “approximately” as used herein is inclusive of the statedvalue and means within an acceptable range of deviation for theparticular value as determined by one of ordinary skill in the art,considering the measurement in question and the error associated withmeasurement of the particular quantity (i.e., the limitations of themeasurement system). For example, “about” can mean within one or morestandard deviations, or within ±20%, 10% or 5% of the stated value.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and thepresent disclosure, and will not be interpreted in an idealized oroverly formal sense unless expressly so defined herein.

Hereinafter, a solid electrolyte, a method of preparing the same, and asecondary battery including the solid electrolyte will be described.

A solid electrolyte according to an embodiment includes an inorganiclithium ion conductive film and a porous layer on a surface of theinorganic lithium ion conductive film, wherein the porous layercomprises a first porous layer and a second porous layer, and the secondporous layer is disposed between the inorganic lithium ion conductivefilm and the first porous layer, wherein the first porous layer has apore size which is greater than a pore size of the second porous layer.

The first porous layer, which is on the second porous layer and oppositethe inorganic lithium ion conductive film, may have a greater porositythan a porosity of the second porous layer, which is located between thefirst porous layer and the inorganic lithium ion conductive film. Theporous layer may have a structure in which a pore size graduallyincreases in a thickness direction of the solid electrolyte. The poresize may increase linearly or non-linearly in the thickness direction.More specifically, the pore size of the porous layer gradually increasesin a direction from the surface of the inorganic lithium ion conductivefilm to the outer surface of the porous layer. When a porosity gradientor a pore size gradient of the solid electrolyte is as described above,the solid electrolyte may have improved ionic conductivity andmechanical strength. As used herein, the term “thickness direction”refers to a direction from the inorganic lithium ion conductive film 11towards the opposite surface of the solid electrolyte, as shown in FIG.1.

An inorganic solid electrolyte may be used as an electrolyte in alithium secondary battery including a lithium metal electrode. When theinorganic solid electrolyte is used, lithium penetration into a grainboundary of the inorganic solid electrolyte may occur. As a result,lithium ion conductivity may decrease at the grain boundary, a shortcircuit may occur due to lithium plating on or in the grain boundary,and resistance of the battery may be increased due to interfacialresistance between the lithium metal electrode and the solidelectrolyte.

The inventors have developed a solid electrolyte including a porouslayer located on at least one surface thereof, wherein a pore size atthe surface of the porous layer is greater than a pore size inside theporous layer. In an embodiment, a region of the porous layer located onthe surface of the solid electrolyte may have a greater porosity thananother region of the porous layer located inside the solid electrolyte.In the solid electrolyte, when the pore size and the porosity at thesurface are greater than in the interior of the porous layer asdescribed above, a contact area between an electrode in contact with thesolid electrolyte and the solid electrolyte increases, thereby reducinginterfacial resistance. Thus, since the surface of the solid electrolytehas a porous structure with a large surface area, stress ofelectrodeposited lithium metal, e.g., lithium electrodeposited at a highcurrent density, may not be concentrated in a localized area of thesolid electrolyte, e.g., at a defect. In addition, lithium penetrationinto a grain boundary may be inhibited.

A structure of a solid electrolyte according to an embodiment will befurther described with reference to FIGS. 1 and 2.

A solid electrolyte 10 shown in FIG. 1 includes an inorganic lithium ionconductive film 11 and a porous layer 14 located thereon. The porouslayer 14 includes a first porous layer 13 located on the surface of thesolid electrolyte 10 and a second porous layer 12 located between theinorganic lithium ion conductive film 11 and the first porous layer 13.A pore size of the first porous layer 13 is greater than a pore size ofthe second porous layer 12. A porosity of the first porous layer 13 maybe greater than a porosity of the second porous layer 12. The inorganiclithium ion conductive film 11 of the solid electrolyte 10, excludingthe porous layer 14, forms a dense portion, has ionic conductivity,blocks movement of substances other than ions between a positiveelectrode and a negative electrode, and inhibits penetration of alithium dendrite. In FIG. 1, an arrow indicates a thickness direction ofthe solid electrolyte 10.

A solid electrolyte 10 of FIG. 2 is the same as the solid electrolyte 10of FIG. 1, except that a third porous layer 15 having a smaller poresize than the pore size of the second porous layer 12 is located betweenthe second porous layer 12 and the lithium ion conductive film 11. Apore size of the first porous layer 13 is greater than the pore size ofthe second porous layer 12.

A porosity of the third porous layer 15 may be adjusted to be smallerthan that of the second porous layer 12 and the porosity of the secondporous layer 12 may be adjusted to be smaller than that of the firstporous layer 13.

The solid electrolyte 10 may have a structure in which a pore sizegradually increases in the thickness direction. The porous layers of thesolid electrolyte 10 of FIG. 1 and the solid electrolyte 10 of FIG. 2have a double-layer structure and a triple-layer structure,respectively. The solid electrolyte may also have a multilayer structureincluding at least three layers.

In FIG. 1, the first porous layer 13 may comprise, consist essentiallyof, or consist of a macropore and the second porous layer 12 maycomprise, consist essentially of, or consist of a micropore. As usedherein, the term “macropore” refers to a pore having a diameter, or amaximum dimension such as a length or width dimension, of greater thanor equal to about 10 micrometers, e.g., 10 micrometers to 1000micrometers, and the term “micropore” refers to a pore having adiameter, or a maximum dimension such as a length or width dimension, ofless than about 10 micrometers, e.g., 0.01 to 10 micrometers. Although athickness ratio of the first porous layer 13 to the second porous layer12 is not particularly limited, the thickness ratio may be from about0.1:1 to about 1:1, or about 0.2:1 to about 1:1, or about 0.5:1 to about1:1.

The first porous layer 13 may include open pores. When the first porouslayer 13 includes open pores, a surface area of the solid electrolyteeffectively increases, thereby improving adhesion between the solidelectrolyte 10 and an electrode. The second porous layer 12 may includeclosed pores. When the second porous layer 12 includes closed pores,ionic conductivity of the solid electrolyte may increase.

A total thickness and density of the porous layers located on thesurface of and inside of the solid electrolyte, may be selected toprevent penetration of a liquid through the solid electrolyte. In anembodiment, the porous layer has a total thickness and density such thatthe solid electrolyte is impermeable to a liquid. A total thickness ofthe porous layer may be from about 5% to about 95%, about 10% to about90%, or about 15% to about 85%, of the total thickness of the solidelectrolyte. When the percentage of the porous layer of the totalthickness of the solid electrolyte is within this range, the relativelydense portion of the solid electrolyte (i.e., i) the region of the solidelectrolyte proximate to the inorganic lithium ion conductive film orii) region of the solid electrolyte excluding the porous layer) maycompletely prevent the penetration of a dendrite of an electrodecomponents, e.g., lithium, and the number of ion conduction paths formedin the porous layer may increase battery capacity and increase outputpower.

The total thickness of solid electrolyte may be from about 10micrometers (μm) to about 500 μm, or about 30 μm to about 400 μm, or forexample, from about 40 μm to about 300 μm. The thickness of the porouslayer 14 may be from about 1 μm to about 475 μm, or about 5 μm to about250 μm, or about 10 μm to about 100 μm, or for example, from about 10 μmto about 30 μm.

The porous layer 14 of the solid electrolyte 10 shown in FIG. 1 may havean average pore size of about 0.1 μm to about 1,000 μm, for example,about 0.1 μm to about 500 μm, for example, about 1 μm to about 50 μm, orfor example, about 1 μm to about 35 μm. The first porous layer 13located on the surface of the solid electrolyte 10 may have an averagepore size of about 10 μm to about 500 μm, or about 10 μm to about 100μm, or for example, about 15 μm to about 35 μm. The second porous layer12 located inside of the solid electrolyte 10 may have an average poresize of about 0.1 μm to about 10 μm, or about 0.5 μm to about 7.5 μm, orfor example, about 1 μm to about 5 μm.

The porous layer 14 may have a total porosity of about 5% to about 70%.The first porous layer 13 located on the surface of the solidelectrolyte 10 may have a porosity of about 30% to about 80%, or about40% to about 75%, or about 50% to about 75%, and the second porous layer12 located inside the solid electrolyte 10 may have a porosity of about1% to about 40%, or about 1% to about 30%, or about 1% to about 25%.

The first porous layer 13 may include an open pores that is open to theoutside of the porous layer 14. The first porous layer 13 may have anopen porosity of about 30% or greater, or about 35% or greater, or about40% or greater, for example, about 30% to about 90%, or about 30% toabout 50%, or about 50% to about 90%. The open porosity of the firstporous layer 13 refers to a percentage of the volume of open pores inthe first porous layer 13 relative to the total pore volume of the firstporous layer 13. When the open porosity of the first porous layer 13 isabout 30% or greater, intrusion of an electrode active material into thefirst porous layer 13, in the case where an electrode active materiallayer is formed on the first porous layer 13, may be improved. As aresult, a contact area between an electrode active material and thefirst porous layer 13 may increase, thereby further increasing batterycapacity. In addition, the open porosity of the first porous layer 13may be from about 60% to about 100%, for example, from about 70% toabout 100%, or for example, from about 80% to about 100%. When the openporosity is within these ranges, the majority of the pores formed in thefirst porous layer 13 may be open pores.

In the solid electrolyte 10 of FIG. 2, the porous layer 14 may have anaverage pore size of about 0.1 μm to about 1,000 μm, for example, about0.1 μm to about 500 μm, for example, about 1 μm to about 50 μm, or forexample, about 1 μm to about 35 μm. The porous layer 14 also has a widepore size distribution. In addition, the first porous layer 13 locatedon the surface of the solid electrolyte 10 may have an average pore sizeof about 10 μm to about 500 μm, or about 10 μm to about 300 μm, or forexample, about 15 μm to about 35 μm, the second porous layer 12 locatedinside the solid electrolyte 10 may have an average pore size of about0.1 μm to about 10 μm, or about 0.5 μm to about 7.5 μm, or for example,about 1 μm to about 5 μm, and the third porous layer 15 may have anaverage pore size of about 0.1 μm to about 5 μm, or about 0.1 μm toabout 3 μm, or for example, about 0.1 μm to about 1 μm. The porous layer14 may have a porosity of about 5% to about 60%, and the first porouslayer 13 located on the surface of the solid electrolyte 10 may have aporosity of about 5% to about 80%, and the second porous layer 12 andthe third porous layer 15 located inside the solid electrolyte 10 mayhave a porosity of about 1% to about 50% and about 1% to about 30%,respectively.

The porous layer may have an irregular or a regular porous structure.The term “irregular porous structure” refers to a structure includingpores having non-uniform pore sizes and non-uniform shapes.

As used herein, the term “pore size” or “average pore size” refers to anaverage diameter of pores when the pores are spherical or circular. Whenthe pores have an oval shape, the pore size refers to a length along amajor axis. As used herein the term “porosity” is used to refer to ameasure of the empty space (i.e., voids or pores) in a material and isdetermined as a percentage of the volume of voids in a material based onthe total volume of the material. The pore size and porosity may bedetermined based on a cross-section scanning electron microscopy imageor by a Brunauer, Emmett and Teller (BET) method. The open porosity maybe calculated based on, for example, bulk density and sintered densityas measured by the Archimedes method.

As used herein, the term “surface of the solid electrolyte” refers to anoutermost surface of the solid electrolyte, which extends a distance ofabout 5% to about 95%, for example, about 1% to about 40%, of a totaldistance from the surface of the solid electrolyte to the center of thesolid electrolyte, or area region within a distance of about 40 μm fromthe outermost edge of the solid electrolyte.

The term “inside of the solid electrolyte” refers to the portion of thesolid electrolyte directly below and disposed continuously along thesurface of the solid electrolyte, and corresponds to a region extendingabout 5% to about 100%, for example, about 60% to about 100% of a totaldistance from the outermost surface of the solid electrolyte, or theremaining region excluding the surface of the solid electrolyte, orexcluding the region within 40 μm from the outermost edge of the solidelectrolyte.

In an embodiment, the porous layer may be permeable to liquid. In anembodiment a dense portion of the solid electrolyte, i.e., the portionwhich is not acid-treated, is not permeable to liquid.

At least one portion of the solid electrolyte 10 of FIG. 1 may includean inorganic lithium ion conductor including a lithium ion, and aportion of the lithium ions is substituted by a proton. The first porouslayer 13 located on the surface of the solid electrolyte 10 of FIG. 1may include a first inorganic lithium ion conductor substituted withabout 2% to about 100%, or about 5% to about 95%, or about 25% to about75% of protons, and the second porous layer 12 inside of the solidelectrolyte 10 may include a second inorganic lithium ion conductorsubstituted with about 0.01% to about 20%, or about 0.05% to about 15%,or about 0.1% to about 10%, of protons.

The porous layer 14 may be a product obtained by acid-treating aninorganic lithium ion conductive film with an acid having aconcentration of greater than or equal to about 0.1 molar (M) and lessthan or equal to about 5 M. Alternatively, the porous layer 14 may be aproduct obtained by forming a multilayer film on an inorganic lithiumion conductive film and heat-treating the multilayer film. The formingmay comprise coating a first composition including a pore former on asurface of the inorganic lithium ion conductive film and coating asecond composition including a pore former on the first composition,wherein the amount of the pore former in the first composition isdifferent from the amount of the pore former in the second composition.

The concentration of the acid may be 0.5 M to 5 M, for example, 0.5 M to4.5 M, or for example, 1 M to 3 M. When the concentration of the acid isless than 0.1 M, lithium carbonate may remain in the solid electrolyte.On the contrary, when the concentration of the acid is 5 M or greater,the surface of the solid electrolyte may be damaged, making it difficultto manufacture a solid electrolyte including a porous layer havingdesired pore size and porosity.

By acid-treating the inorganic lithium ion conductive film or by formingthe multilayer film including the pore former and heat-treating themultilayer film, a portion of the inorganic lithium ion conductor may beselectively dissolved by the acid or the pore former to form a porousstructure. When the porous structure is formed as described above,lithium carbonate is removed from the solid electrolyte up to asub-surface and a lithium ion of the lithium ion conductor may besubstituted with a proton. Also, crystallinity of the solid electrolyteis improved and an average grain size is increased. As a result,formation of a metallic compound, which may cause a short circuit in thesecondary battery, may be prevented and concentration of current may beinhibited thereby suppressing a short circuit at a high current density.In addition, due to the presence of the protonated inorganic lithium ionconductor and the increased specific surface area at the surface of thesolid electrolyte, interfacial resistance with a lithium metal electrodemay be considerably reduced.

A lattice constant (e.g., lattice distance) of the porous layer of thesolid electrolyte, as measured by X-ray diffraction (XRD), is greaterthan a lattice constant of the remaining area of the solid electrolyteexcluding the porous layer by about 0.005 angstrom (Å) to about 0.1 Å,or about 0.001 Å to about 0.1 Å, or about 0.01 Å to about 0.1 Å. Theaverage grain size of the porous layer is at least two times greaterthan the average grain size of the remaining area of the solidelectrolyte excluding the porous layer. The average grain size of thesolid electrolyte may be, for example, from about 200 nm to about 300 nmand the lattice constant thereof may be from about 12.96 Å to about12.98 Å. A “grain” as used herein means a particle or region of aparticle having a single crystallographic orientation.

For example, the inorganic lithium ion conductive film of the solidelectrolyte may include at least one of a garnet compound, an argyroditecompound, a lithium super-ion-conductor (LISICON) compound, a Na superionic conductor-like (NASICON) compound, a Li nitride, a Li hydride, aperovskite, or a Li halide.

The inorganic lithium ion conductive film may include at least one of agarnet ceramic (Li_(3+x)La₃M₂O₁₂, wherein 0≤x≤5, M is at least one of W,Ta, Te, Nb, or Zr), a doped garnet ceramic (Li_(3+x)La₃M₂O₁₂, wherein0≤x≤5, M is at least one of W, Ta, Te, Nb, or Zr),Li_(1+x+y)Al_(x)Ti_(2-x)Si_(y)P_(3-y)O₁₂ (wherein 0<x<2 and 0≤y<3),BaTiO₃, Pb(Zr,Ti)O₃ (PZT), Pb_(1-x)La_(x)Zr_(1-y)Ti_(y)O₃ (PLZT) (0≤x<1and 0≤y<1), Pb(Mg_(1/3)Nb_(2/3))O₃—PbTiO₃ (PMN-PT), lithium phosphate(Li₃PO₄), lithium titanium phosphate (Li_(x)Ti_(y)(PO₄)₃, wherein 0<x<2,and 0<y<3), lithium aluminum titanium phosphate(Li_(x)Al_(y)Ti_(z)(PO₄)₃, wherein 0<x<2, 0<y<1, and 0<z<3),Li_(1+x+y)(Al,Ga)_(x)(Ti,Ge)_(2-x)Si_(y)P_(3-y)O₁₂ (wherein 0≤x≤1 and0≤y≤1), lithium lanthanum titanate (Li_(x)La_(y)TiO₃, wherein 0<x<2, and0<y<3), lithium germanium thiophosphate (Li_(x)Ge_(y)P_(z)S_(w), wherein0<x<4, 0<y<1, 0<z<1, and 0<w<5), lithium nitride (Li_(x)N_(y), wherein0<x<4, and 0<y<2), SiS₂-based glass (Li_(x)Si_(y)S_(z), wherein 0≤x<3,0<y<2, and 0<z<4), P₂S₅-based glass (wherein Li_(x)P_(y)S_(z), 0≤x<3,0<y<3, and 0<z<7), Li_(3x)La_(2/3-x)TiO₃ (0≤x≤16), Li₇La₃Zr₂O₁₂,Li_(1+y)Al_(y)Ti_(2-y)(PO₄)₃ (0≤y≤1), Li_(1+z)Al_(z)Ge_(2-z)(PO₄)₃(wherein 0≤z≤1), Li₂O, LiF, LiOH, Li₂CO₃, LiAlO₂, aLi₂O—Al₂O₃—SiO₂—P₂O₅—TiO₂—GeO₂-based ceramic, Li₁₀GeP₂S₁₂,Li_(3.25)Ge_(0.25)P_(0.75)S₄, Li₃PS₄, Li₆PS₅Br, Li₆PS₅Cl, Li₇PS₅,Li₆PS₅I, Li_(1.3)Al_(0.3)Ti_(1.7)(PO₄)₃, LiTi₂(PO₄)₃, LiGe₂(PO₄)₃,LiHf₂(PO₄)₃, LiZr₂(PO₄)₃, Li₂NH, Li₃(NH₂)₂I, LiBH₄, LiAlH₄, LiNH₂,Li_(0.34)La_(0.51)TiO_(2.94), LiSr₂Ti₂NbO₉,Li_(0.06)La_(0.66)Ti_(0.93)Al_(0.03)O₃, Li_(0.34)Nd_(0.55)TiO₃,Li₂CdCl₄, Li₂MgCl₄, Li₂ZnI₄, Li₂CdI₄,Li_(4.9)Ga_(0.5+δ)La₃Zr_(1.7)W_(0.3)O₁₂ (wherein 0≤δ<1.6),Li_(4.9)Ga_(0.5+δ)La₃Zr_(1.7)W_(0.3)O₁₂ (wherein 1.7≤δ≤2.5), andLi_(5.39)Ga_(0.5+δ)La₃Zr_(1.7)W_(0.3)O₁₂ (wherein 0≤δ<1.11).

In the doped Garnet ceramics Li_(3+x)La₃M₂O₁₂, a doping element includesCe, Pr, Ga, Y or a combination thereof.

The inorganic lithium ion conductive film may include, for example, atleast one compound represented by the Formula 1 or 1a.

Li_(7-x)M¹ _(x)La_(3-a)M² _(a)Zr_(2-b)M³ _(b)O₁₂  Formula 1

Li_(7-x)La_(3-a)M² _(a)Zr_(2-b)M³ _(b)O₁₂  Formula 1a

wherein, in Formula 1, M¹ comprises at least one of gallium (Ga) oraluminum (Al),

in Formulas 1 and 1a, M² comprises at least one of calcium (Ca),strontium (Sr), cesium (Cs), or barium (Ba),

M³ includes at least one of aluminum (Al), tungsten (W), niobium (Nb),or tantalum (Ta), and

0≤x<3, 0≤a≤3, and 0≤b<2.

In Formula 1, x may be from 0.01 to 2.1, for example, 0.01 to 0.99, forexample, from 0.1 to 0.9, and from 0.2 to 0.8. In Formula 1, a may befrom 0.1 to 2.8, for example, 0.5 to 2.75, and b may be from 0.1 to 1,for example, 0.25 to 0.5.

In the compound represented by the Formula 1, a dopant may be at leastone of M¹, M², or M³. In the compound represented by the Formula 1a, adopant may be at least one of M² or M³.

The inorganic lithium ion conductive film may include, for example, atleast one of Li₇La₃Zr₂O₁₂ (LLZO), Li_(6.4)La₃Zr_(1.7)W_(0.3)O₁₂,Li_(6.5)La₃Zr_(1.5)Ta_(0.3)O₁₂, Li₇La₃Zr_(1.7)W_(0.3)O₁₂,Li_(4.9)La_(2.5)Ca_(0.5)Zr_(1.7)Nb_(0.3)O₁₂,Li_(4.9)Ga_(2.1)La₃Zr_(1.7)W_(0.3)O₁₂, Li₇La₃Zr_(1.5)W_(0.5)O₁₂,Li₇La_(2.75)Ca_(0.25)Zr_(1.75)Nb_(0.25)O₁₂, Li₇LaZr_(1.5)Nb_(0.5)O₁₂,Li₇LaZr_(1.5)Ta_(0.5)O₁₂, Li_(6.272)La₃Zr_(1.7)W_(0.3)O₁₂, orLi_(5.39)Ga_(1.61)La₃Zr_(1.7)W_(0.3)O₁₂. In the inorganic lithium ionconductive film, the inorganic lithium ion conductor may have a particlestructure or a columnar structure. In the porous layer, a lithium ion ofthe inorganic lithium ion conductor may be substituted with a protonsuch that an amount of protons in the porous layer may be from about0.01 mole percent (mol %) to about 50 mol %, or about 0.1 mol % to about30 mol %, or for example, about 0.1 mol % to about 20 mol %, of thetotal number of protons and lithium ions.

The surface of the solid electrolyte may includeLi_(7-x)H_(x)La₃Zr_(2-y)M_(y)O₁₂ (wherein 0.1≤x≤7, 0≤y≤2, and M is atleast one of W, Ta, Te, or Nb) and the inside of the solid electrolytemay include Li_(7-x)H_(x)La₃Zr_(2-y)M_(y)O₁₂ (wherein 0≤x≤6.5, 0≤y≤2,and M is at least one of W, Ta, Te, or Nb).

The surface of the solid electrolyte may include a lithium ion conductorsubstituted with Li_((7-x))H_(x)La₃Zr₂O₁₂ (wherein 0.1≤x≤7), and theinside of the solid electrolyte may include Li_((7-x))H_(x)La₃Zr₂O₁₂(wherein 0≤x<6.5).

The surface of the solid electrolyte may include a lithium ion conductorsubstituted with Li_((6.75-x))H_(x)La_(2.9)Ga_(0.1)Nb_(0.25)Zr_(1.75)O₁₂(wherein 0.1≤x<6.75) and the inside thereof may includeLi_((7-x))H_(x)La₃Zr₂O₁₂ (wherein 0≤x<6.5). In this case, in the solidelectrolyte, portions of an inorganic lithium ion conductive film onwhich the porous layer is not formed may includeLi_(6.75)La_(2.9)Ga_(0.1)Nb_(0.25)Zr_(1.75)O₁₂.

In the inorganic lithium ion conductive film, a grain of the inorganiclithium ion conductor may have a polyhedral shape. In the case of agrain having a polyhedral shape, the contact area between adjacentgrains may increase, thereby increasing ion conductivity and increasingthe possibility of contact between an active material and a crystalplane efficient for a charge transfer reaction, thereby increasing thekinetics of an electrochemical reaction.

A secondary battery according to another embodiment includes a positiveelectrode, a negative electrode, and a solid electrolyte interposedbetween the positive electrode and the negative electrode.

Referring to FIG. 3, a secondary battery according to an embodiment willbe described in more detail.

In a secondary battery 28, a positive electrode 27 is located on apositive current collector 26, and a solid electrolyte 20 is located onthe positive electrode 27. A solid electrolyte 20 shown in FIG. 3includes an inorganic lithium ion conductive film 21 and a porous layer24 located thereon. The porous layer 24 of the solid electrolyte 20 isarranged to be adjacent to a negative electrode 29. A negative currentcollector 30 is disposed (e.g., laminated) on the negative electrode 29.

In the porous layer 24, a first porous layer 23 is located on thesurface of the solid electrolyte 20 and a second porous layer 22 havinga smaller pore size than that of the first porous layer 23 is locatedinside of the solid electrolyte 20. An electrolyte may be locatedbetween the positive electrode 27 and the solid electrolyte 20. Theelectrolyte may include at least one of an ionic liquid, a lithium salt,an organic solvent, or a polymer ionic liquid. The electrolyte may beimpregnated in the positive electrode 27.

The ionic liquid, is an ionic material in a molten state at roomtemperature (25° C.), and may be any material including a cation and ananion without limitation. Examples of the ionic liquid may include atleast one compound including at least one cation and at least one anion.The at least one cation may include at least one of ammonium,pyrrolidium, pyridinium, pyrimidium, imidazolium, piperidinium,pyrazolium, oxazolium, pyridazinium, phosphonium, sulfonium, ortriazolium. The at least one anion may include at least one of BF₄ ⁻,PF₆ ⁻, AsF₆ ⁻, SbF₆ ⁻, AlCl₄ ⁻, HSO₄ ⁻, ClO₄ ⁻, CH₃SO₃ ⁻, CF₃CO₂ ⁻, Cl⁻,Br⁻, I⁻, SO₄ ²⁻, CF₃SO₃ ⁻, (FSO₂)₂N⁻, (C₂F₅SO₂)₂N⁻, (C₂F₅SO₂)(CF₃SO₂)N⁻,or (CF₃SO₂)₂N⁻.

For example, the ionic liquid may be at least one of [emim]C/AlCl₃ (emimis ethyl methyl imidazolium), [bmpyr]NTf2 (bmpyr is butyl methylpyridinium), [bpy]Br/AlC₃ (bpy=4,4′-bipyridine), [choline]Cl/CrCl₃.6H₂O,[emim]OTf/[hmim]I (hmim is hexyl methyl imidazolium),[choline]Cl/HOCH₂CH₂OH, [Et₂MeN(CH₂CH₂OMe)]BF₄ (Et is ethyl, Me ismethyl, Pr is propyl, Bu is butyl, Ph is phenyl, Oct is octyl, Hex ishexyl), [Bu₃PCH₂CH₂C₈F₁₇]OTf (OTf is trifluoromethane sulfonate),[bmim]PF₆ (bmim is butyl methyl imidazolium), [bmim]BF₄, [omim]PF₆ (omimis octyl methyl imidazolium), [Oct₃PC₁₈H₃₇]I, [NC(CH₂)₃mim]NTf₂ (mim ismethyl imidazolium), [Pr₄N][B(CN)₄], [bmim]NTf₂, [bmim]Cl,[bmim][Me(OCH₂CH₂)₂OSO₃], [PhCH₂mim]OTf, [Me₃NCH(Me)CH(OH)Ph]NTf₂,[pmim][(HO)₂PO₂] (pmim is propyl methyl imidazolium), [(6-Me)bquin]NTf₂(bquin is butyl quinolinium, [bmim][Cu₂Cl₃], [C₁₈H₃₇OCH₂mim]BF₄ (mim ismethyl imidazolium), [heim]PFe (heim is hexyl ethyl imidazolium),[mim(CH₂CH₂O)₂CH₂CH₂mim][NTf₂]₂ (mim is methyl imidazolium), [obim]PF₆(obim is octyl butyl imidazolium), [oquin]NTf₂ (oquin is octylquinolinium), [hmim][PF₃(C₂F₅)₃], [C₁₄H₂₉mim]Br (mim is methylimidazolium), [Me₂N(C₁₂H₂₅)₂]NO₃, [emim]BF₄, [MeN(CH₂CH₂OH)₃], [MeOSO₃],[Hex₃PC₁₄H₂₉]NTf₂, [emim][EtOSO₃], [choline][ibuprofenate], [emim]NTf₂,[emim][(EtO)₂PO₂], [emim]Cl/CrCl₂, [Hex₃PC₁₄H₂₉]N(CN)₂, and the like.However, the ionic liquid is not limited thereto and any suitable ionicliquid may also be used therefor.

Unless specified otherwise, mim is methyl imidazolium, emim is ethylmethyl imidazolium, hmim is hexyl methyl imidazolium, obim is octylbutyl imidazolium, bmim is butyl methyl imidazolium, omim is octylmethyl imidazolium, pmim is propyl methyl imidazolium, bppyr is butylmethyl pyridinium, bpy is 4,4′-bipyridine, Et is ethyl, Me is methyl, Pris propyl, Bu is butyl, Ph is phenyl, Oct is octyl, Hex is hexyl, py ispyridine, obim is octyl butyl imidazolium, bquin is butyl quinolinium,heim is hexyl ethyl imidazolium, oquin is octyl quinolinium, OTf istrifluoromethane sulfonate, and NTf₂ isbis(trifluoromethanesulfonyl)imide.

The lithium salt may include at least one of LiSCN, LiN(CN)₂, LiClO₄,LiBF₄, LiAsF₆, LiPF₆, LiCF₃SO₃, LiC(CF₃SO₂)₃, LiN(SO₂C₂F₅)₂,LiN(SO₂CF₃)₂, LiN(SO₂F)₂, LiSbF₆, LiPF₃(CF₂CF₃)_(a), LiPF₃(CF₃)₃, orLiB(C₂O₄)₂. In addition, a concentration of the lithium salt is fromabout 1 M to about 6 M, or for example, from about 1.5 M to about 5 M.

The negative electrode 29 may be a lithium metal negative electrode.

The solid electrolyte 20 and the negative electrode 29 may be bonded toeach other by a method known to those of skill in the art. For example,the solid electrolyte and the negative electrode may be bonded to eachother by cold isostatic pressing (CIP). According to another embodiment,the solid electrolyte 20 and the negative electrode 29 may be bonded viaa molten Li or a polyethylene oxide binder.

The secondary battery 28 may further include at least one of a liquidelectrolyte, a polymer electrolyte, or a lithium salt.

The liquid electrolyte may include a lithium salt and an organicsolvent.

The organic solvent may include an aprotic solvent or protic solvent.Examples of the aprotic solvent may include carbonate-based,ester-based, ether-based, or ketone-based solvents. The protic solventmay be include alcohol-based solvents. Examples of the carbonate-basedsolvent may include dimethyl carbonate (DMC), diethyl carbonate (DEC),ethyl methyl carbonate (EMC), dipropyl carbonate (DPC), methyl propylcarbonate (MPC), ethyl propyl carbonate (EPC), ethylene carbonate (EC),propylene carbonate (PC), and butylene carbonate (BC). Examples of theester-based solvents may include methyl acetate, ethyl acetate, n-propylacetate, t-butyl acetate, methyl propionate, ethyl propionate,γ-butyrolactone, decanolide, valerolactone, mevalonolactone, andcaprolactone. Examples of the ether-based solvents may include dibutylether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran,tetrahydrofuran, and tetraethylene glycol dimethyl ether (TEGDME).Examples of the ketone-based solvent may include cyclohexanone. Also,examples of the alcohol-based solvent may include ethyl alcohol andisopropyl alcohol. However, examples of the aprotic solvent are notlimited thereto, and any material available as an aprotic solvent in theart may also be used.

The polymer electrolyte may be any polymer electrolyte suitable for usein a secondary battery.

A solid sulfide electrolyte may further be located between the negativeelectrode and the solid electrolyte. For example, the solid sulfideelectrolyte may be Li₂S—P₂S₅—LiX (wherein X is at least one of F, Cl,Br, or I).

The secondary battery may be configured to operate at a current densityof 3 milliamperes per square centimeter (mA/cm²) over 1,000 cycles, forexample, over 1,700 cycles.

A lifespan of the secondary battery is greater than 1,700 cycles whenthe secondary battery has a capacity of 80% of an initial capacity.

The negative electrode may be a lithium metal electrode or a lithiummetal alloy electrode. A surface of the solid electrolyte in contactwith the negative electrode has a greater pore size than a portion ofthe solid electrolyte furthest away from the negative electrode.

The secondary battery may be a lithium metal battery including anegative electrode including at least one of a lithium metal or alithium metal alloy.

In a lithium metal battery including a lithium metal negative electrode,the solid electrolyte according to an embodiment may serve as aprotective layer to protect the lithium metal negative electrode. Thesolid electrolyte according to an embodiment and a lithium metalnegative electrode together may constitute a protective negativeelectrode.

Hereinafter, a method of preparing a secondary battery according to anembodiment will be described.

A positive electrode is prepared according to the following method.

A positive active material, a binder, and a solvent are mixed to preparea positive active material composition.

A conductive agent may further be added to the positive active materialcomposition.

The positive active material composition may be directly coated on ametal current collector and dried to prepare a positive electrode plate.Alternatively, the positive active material composition may be cast on aseparate support, and then a film separated from the support may belaminated on a metal current collector to prepare a positive electrodeplate.

The binder facilitates the binding of the positive active material tothe conductive agent, the current collector, and the like. Examples ofthe binder may include, but are not limited to, at least one ofpolyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose,polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene,ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrenebutadiene rubber (SBR), fluorine rubber, or a copolymer thereof.

The conductive agent may be any compound having electrical conductivitywithout causing a chemical change. For example, the conductive agent maybe at least one of graphite such as natural graphite and artificialgraphite; a carbonaceous material such as carbon black, acetylene black,Ketjen black, channel black, furnace black, lamp black, and thermalblack; a conductive fiber such as carbon fiber and metal fiber; carbonfluoride; metal powder such as aluminum powder and nickel powder;conductive whisker such as zinc oxide and potassium titanate; conductivemetal oxide such as titanium oxide; or conductive materials such aspolyphenylene derivatives may be used as the conductive agent. Acombination comprising at least one of the foregoing may also be used.

The solvent may be N-methylpyrrolidone (NMP), without being limitedthereto.

Amounts of the positive active material, the conductive agent, thebinder, and the solvent may be determined by those of skill in the artwithout undue experimentation. At least one of the conductive agent, thebinder, or the solvent may not be used according to the use and thestructure of the lithium battery.

The positive active material used to prepare the positive electrode mayinclude at least one of lithium cobalt oxide, lithium nickel cobaltmanganese oxide, lithium nickel cobalt aluminum oxide, lithium ironphosphate, or lithium manganese oxide, without being limited thereto,and, any suitable positive active material may also be used.

For example, the positive active material may include at least one ofthe following: Li_(a)A_(1-b)B′_(b)D₂ (where 0.90≤a≤1.8 and 0≤b≤0.5);Li_(a)E_(1-b)B′_(b)O_(2-c)D_(c) (where 0.90≤a≤1.8, 0≤b≤0.5, and0≤c≤0.05); LiE_(2-b)B′_(b)O_(4-c)D_(c) (where 0≤b≤0.5 and 0≤c≤0.05);Li_(a)Ni_(1-b-c)CO_(b)B′_(c)D_(α) (where 0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05,and 0<α<2); Li_(a)Ni_(1-b-c)Co_(b)B′_(c)O_(2-α)F′_(α) (where 0.90≤a≤1.8,0≤b≤0.5, 0≤c≤0.05, and 0<α<2); Li_(a)Ni_(1-b-c)Mn_(b)B′_(c)D_(α) (where0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, and 0<α≤2);Li_(a)Ni_(1-b-c)Mn_(b)B′_(c)O_(2-α)F_(α) (where 0.90≤a≤1.8, 0≤b≤0.5,0≤c≤0.05, and 0<α<2); Li_(a)Ni_(b)E_(c)G_(d)O₂ (where 0.90≤a≤1.8,0≤b≤0.9, 0≤c≤0.5, and 0.001≤d≤0.1); Li_(a)Ni_(b)Co_(c)Mn_(d)GeO₂ (where0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5, 0≤d≤0.5, and 0.001≤e≤0.1); Li_(a)NiG_(b)O₂(where 0.90≤a≤1.8 and 0.001≤b≤0.1); Li_(a)CoG_(b)O₂ (where 0.90≤a≤1.8and 0.001≤b≤0.1); Li_(a)MnG_(b)O₂ (where 0.90≤a≤1.8 and 0.001≤b≤0.1);Li_(a)Mn₂G_(b)O₄ (where 0.90≤a≤1.8 and 0.001≤b≤0.1); QO₂; QS₂; LiQS₂;V₂O₅; LiV₂O₅; LiI′O₂; LiNiVO₄; Li_((3-f))J₂(PO₄)₃ (0≤f≤2);Li_((3-f))Fe₂(PO₄)₃ (0≤f≤2); or LiFePO₄.

In these formulae above, A is nickel (Ni), cobalt (Co), manganese (Mn),or any combination thereof; B′ is aluminum (Al), nickel (Ni), cobalt(Co), manganese (Mn), chromium (Cr), iron (Fe), magnesium (Mg),strontium (Sr), vanadium (V), a rare earth element or any combinationthereof; D is oxygen (O), fluorine (F), sulfur (S), phosphorus (P), orany combination thereof; E is cobalt (Co), manganese (Mn), or anycombination thereof; F′ is fluorine (F), sulfur (S), phosphorus (P), orany combination thereof; G is aluminum (Al), chromium (Cr), manganese(Mn), iron (Fe), magnesium (Mg), lanthanum (La), cerium (Ce), strontium(Sr), vanadium (V), or any combination thereof; Q is titanium (Ti),molybdenum (Mo), manganese (Mn), or any combination thereof; I′ ischromium (Cr), vanadium (V), iron (Fe), scandium (Sc), yttrium (Y), orany combination thereof; and J is vanadium (V), chromium (Cr), manganese(Mn), cobalt (Co), nickel (Ni), copper (Cu), or any combination thereof.

For example, the positive active material may be at least one of thecompounds represented by Formulae 1, 2, or 3 below.

Li_(a)Ni_(b)Co_(c)Mn_(d)O₂  Formula 1

In Formula 1, 0.90≤a≤1.8, 0≤b≤0.95, 0≤c≤0.5, and 0≤d≤0.5.

Li₂MnO₃  Formula 2

LiMO₂  Formula 3

In Formula 3, M is Mn, Fe, Co, or Ni.

The lithium secondary battery may further include at least one of aliquid electrolyte, a solid electrolyte, a gel electrolyte, or a polymerionic liquid.

The negative electrode may be a lithium metal negative electrodeincluding at least one of a lithium metal or a lithium metal alloy ormay include a negative active material including at least one of acarbonaceous material, silicon, silicon oxide, a silicon alloy, asilicon-carbon composite, tin, a tin alloy, a tin-carbon composite, ametal/metalloid alloyable with lithium, an alloy thereof, or an oxidethereof.

The negative electrode may be at least one of a lithium metal thin filmor a lithium metal alloy thin film.

The lithium metal alloy may include Li and a metal/metalloid alloyablewith Li. For example, the metal/metalloid alloyable with Li may be Si,Sn, Al, Ge, Pb, Bi, Sb, an Si—Y′ alloy (Y′ is at least one of an alkalimetal, an alkali earth metal, a Group 13 to 16 element, a transitionmetal, a rare earth element, except for Si), an Sn—Y′ alloy (Y′ is atleast one of an alkali metal, an alkali earth metal, a Group 13 to 16element, a transition metal, or a rare earth element, except for Sn). Inthis regard, Y′ may be at least one of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti,Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os,Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As,Sb, Bi, S, Se, Te, or Po.

The solid electrolyte according to an embodiment is located on thenegative electrode.

A secondary battery may be prepared by accommodating the positiveelectrode in a battery case, injecting an electrolyte thereinto, andlocating the positive electrode on one surface of the solid electrolyte,the other surface of which is mounted with the negative electrode.

The electrolyte may include at least one of a lithium salt, an ionicliquid, a polymer ionic liquid, or an organic solvent. For example, theelectrolyte includes a lithium salt and an ionic liquid.

The secondary battery may be, for example, a lithium secondary batteryin which a lithium electrode is used as a negative electrode, a lithiumair battery in which a lithium electrode is used as a negative electrodeand oxygen is used as a positive electrode, a lithium sulfur battery inwhich a lithium electrode is used as a negative electrode and a positiveelectrode includes sulfur or a sulfur-containing active material, or thelike.

The secondary battery according to an embodiment may include at leastone of sodium, magnesium, calcium, or aluminum in addition to lithium asa material used to form the negative electrode.

The secondary battery including the solid electrolyte according to anembodiment may be, for example, an all-solid-state secondary battery ora lithium ion secondary battery.

Surface structures and components of the solid electrolyte according toan embodiment may be analyzed by X-ray diffraction (XRD), scanningelectron microscopy (SEM), transmission electron microscopy (TEM),inductively coupled plasma (ICP) analysis, X-ray photoelectronspectroscopy (XPS), or the like.

A method of preparing the solid electrolyte according to anotherembodiment includes acid-treating an inorganic lithium ion conductivefilm by applying an acid having a concentration of greater than or equalto about 0.1 M and less than 5 M thereto; and cleaning an acid-treatedproduct. In particular, the method includes a first acid treatmentcomprising acid-treating an inorganic lithium ion conductive film withan acid having a concentration of greater than or equal to about 0.1molar and less than or equal to about 5 molar to provide a firstacid-treated product, and a first cleaning comprising cleaning the firstacid-treated product to provide a cleaned first acid-treated productforming the solid electrolyte.

The acid treatment may be performed by dipping a portion of theinorganic lithium ion conductive film in the acid or by disposing a maskon the inorganic lithium ion conductive film and coating the acid onlyon exposed portion of the inorganic lithium ion conductive film.

Acid treatment time may vary in accordance with the concentration of theacid and may be, for example, from about 0.1 hour to about 2 hours.Also, the acid treatment is performed under dry-air conditions (e.g., arelative humidity of 0%) to minimize a reaction between the surface ofthe solid electrolyte and moisture in the air.

The pore size, porosity, and thickness of the porous layer of the solidelectrolyte may vary in accordance with the concentration of the acidduring the acid treatment and acid treatment time.

In a portion of the acid-treated surface, or all of the acid treatedsurface, of the solid electrolyte (e.g., Li₇La₃Zr₂O₁₂ layer), a lithiumion is substituted with a proton and thus lithium carbonate is removedfrom the surface as shown in Reaction Scheme 1 below.

In Reaction Scheme 1, 0<x≤7.

When the portion of the solid electrolyte substituted with a proton isin contact with an acid over a predetermined period of time, dissolutionof the solid electrolyte may occur by decomposition of the solidelectrolyte. In this dissolution, a grain having a smaller size and arelatively high energy state is dissolved selectively from grainboundaries at the surface of the electrolyte layer. Since pores areformed in positions where the grain having a small size is completelydissolved, a porous layer is formed on the surface of the solidelectrolyte. Thickness, porosity, and pore size of the porous layerslocated on the surface of the solid electrolyte and inside of the solidelectrolyte may be adjusted by varying the acid treatment conditions,for example, the concentration of the acid and acid treatment time.

The cleaning may be performed by using, for example, ethanol.

The inorganic lithium ion conductive film may be prepared by combiningan inorganic lithium ion conductor and a lithium compound to prepare amixture, and heat-treating the mixture.

An inorganic lithium ion conductor is prepared by a solid phase methodor liquid phase method by using a precursor for forming the inorganiclithium ion conductor. The inorganic lithium ion conductor and a lithiumcompound are combined to prepare a mixture and the mixture is processedin the form of pellets, sheets, or films. The product in the form ofpellets or sheets is heat-treated. A heat treatment temperature may varyin accordance with type and amount of the inorganic lithium ionconductor and the lithium compound and may be, for example, from about25° C. to about 80° C.

The heat-treated product is polished to remove impurities remaining onthe surface or generated during heat treatment. The polishing may beperformed by using an auto polisher.

The lithium compound is a material capable of supplying lithium to theinorganic lithium ion conductor. Examples of the lithium compound mayinclude at least one of lithium carbonate, lithium hydroxide, or lithiumoxide. The amount of the lithium compound is in the range of about 0.1parts by weight to about 10 parts by weight, for example, about 1 partby weight to about 8 parts by weight, or for example, about 3 parts byweight to about 5 parts by weight based on 100 parts by weight, based ona total weight of the inorganic lithium ion conductor and the lithiumcompound. When the amount of the lithium compound is within the aboveranges, a solid electrolyte including a porous layer having desired poresize and porosity may be obtained. By adding an excess of the lithiumcompound during the preparation of the solid electrolyte, a porous layerhaving a predetermined thickness may be formed without structuralcollapse during acid treatment with a strong acid.

The method further includes a second acid treatment comprisingacid-treating the cleaned first acid-treated product with an acid havinga concentration of greater than or equal to about 0.1 molar and lessthan or equal to about 5 molar to provide a second acid-treated product,and a second cleaning comprising cleaning the second acid-treatedproduct. For example, a second acid treatment is performed by applyingan acid having a concentration of more than 0.1 M and less than 5 Mthereto and a second cleaning process of cleaning a product obtained bythe second acid treatment may be repeatedly performed.

The concentration of the acid used in the second acid treatment is lessthan the concentration of the acid used in the previous (e.g., first)acid treatment.

The cleaned product is dried. The drying is performed, for example, atabout 25° C. to about 60° C.

The acid may include at least one of HCl, H₃PO₄, HNO₃, H₂SO₄, or aceticacid, but is not limited thereto.

A method of preparing the solid electrolyte according to anotherembodiment includes: forming a multilayer film on a surface of aninorganic lithium ion conductive film, the forming including coating afirst composition comprising a pore former and a lithium ion conductoron a surface of the inorganic lithium ion conductive film, and dryingthe first composition, and coating a second composition comprising apore former and a lithium ion conductor on the dried first composition,and drying the second composition to form the multilayer film; andheat-treating the multilayer film. The amount of the pore former in thefirst composition may be different from the amount of the pore formed inthe second composition.

The pore former may be any material capable of forming pores in themultilayer. Examples of the pore former may include at least one ofdibutyl phthalate, dimethyl phthalate, diethyl phthalate, dioctylsebacate, or dioctyl adipate.

Hereinafter, one or more embodiments will be described in detail withreference to the following examples and comparative examples. However,these examples and comparative examples are not intended to limit thepurpose and scope of the one or more embodiments.

Examples Preparation Example 1: Preparation of LLZO Film

Li₇La₃Zr₂O₁₂ (LLZO) powder was obtained by a solid phase method. TheLi₇La₃Zr₂O₁₂ (LLZO) powder was prepared in the same manner as describedin Example 1 of US Patent Application Publication No. 2016/0149260,incorporated herein by reference in its entirety, except that Li₂CO₃,LiOH, La₂O₃, and ZrO₂ were used as oxide precursors.

The Li₇La₃Zr₂O₁₂ (LLZO) powder and lithium carbonate (Li₂CO₃) wereuniaxially pressed at a pressure of about 10 millipascals (MPa) to forma layer. An amount of the lithium carbonate was 5.5 parts by weightbased on 100 parts by weight of a total weight of the Li₇La₃Zr₂O₁₂(LLZO) powder and the lithium carbonate (Li₂CO₃).

Next, the layer was covered with mother powder (Li⁷La₃Zr₂O₁₂) andheat-treated at 1300° C. for 4 hours. A surface of the heat-treatedlayer was polished using an auto polisher to prepare an LLZO Film(thickness: about 300 μm).

Preparation Example 2: Preparation of LLZO Film

An LLZO film was prepared in the same manner as in Preparation Example1, except that Li_(6.4)La₃Zr_(1.7)W_(0.3)O₁₂ (W-doped LLZO) powder wasused instead of the Li₇La₃Zr₂O₁₂ (LLZO) powder.

Preparation Example 3: Preparation of LLZO Film

An LLZO film was prepared in the same manner as in Preparation Example1, except that Li_(6.5)La₃Zr_(1.5)Ta_(0.3)O₁₂ (Ta-doped LLZO) powder wasused instead of the Li₇La₃Zr₂O₁₂ (LLZO) powder.

Preparation Examples 4 and 4a: Preparation of LLZO Film

LLZO films were prepared in the same manner as in Preparation Example 1,except that lithium carbonate was used in amounts of 0.1 parts by weightand 10 parts by weight based on 100 parts by weight of the total weightof Li₇La₃Zr₂O₁₂ (LLZO) powder and lithium carbonate (Li₂CO₃).

Preparation Example 5: Preparation of LLZO Film

An LLZO film was prepared in the same manner as in Preparation Example1, except that lithium hydroxide was used instead of the lithiumcarbonate during the process of uniaxial pressing, and the uniaxialpressing of the Li₇La₃Zr₂O₁₂ (LLZO) powder and lithium hydroxide wasconducted at a pressure of about 200 MPa.

Comparative Preparation Example 1: Preparation of LLZO Film

An LLZO film was prepared in the same manner as in Preparation Example1, except that lithium carbonate (Li₂CO₃) was not added to theLi₇La₃Zr₂O₁₂ (LLZO) powder during the process of uniaxial pressing ofthe Li₇La₃Zr₂O₁₂ (LLZO) powder at a pressure of about 10 MPa.

Comparative Preparation Example 1-1

An LLZO film was prepared according to the following method described inEnergy Environ. Sci., 2017, 1568-1575, page 1573, incorporated herein byreference in its entirety.

LLZO powder and poly(methylmethacrylate) (PMMA), as a pore former, wereadded to a mixed solvent of toluene and isopropanol, and the mixture wasmixed for about 1 hour to prepare a porous layer slurry. Fish oil,polyvinyl butyral, and butyl benzyl phthalate (BBP) were added to theporous layer slurry as a binder and dispersant. The porous layer slurrywas coated on surfaces of Li₇La₃Zr₂O₁₂ (LLZO) pellets by screen printingand heat-treated at about 1100° C. for 2 hours to prepare a porous LLZOfilm.

Example 1: Preparation of Solid Electrolyte

A mask was placed on a surface of the LLZO film prepared according toPreparation Example 1 and a first acid treatment was performed on theLLZO film for 10 minutes by dropping a 1 M HCl aqueous solution onexposed portions of the LLZO film. A product obtained by the first acidtreatment was cleaned using ethanol and dried to form a porous layer ona surface of the LLZO film. The acid treatment and cleaning weresequentially repeated two more times to perform second acid treatmentand third acid treatment, thereby increasing a porosity and a thicknessof the porous layer. The concentration of the acid and the acidtreatment time of the second and third acid treatments were the same asthose of the first acid treatment. The acid treatments were performedunder dry-air conditions (e.g., a relative humidity of 0%) to minimizereaction between the surface of the solid electrolyte and moisture inthe air. A resultant product obtained after cleaning was dried at about25° C. to prepare a solid electrolyte.

Example 2 and Comparative Example 1: Preparation of Solid Electrolyte

Solid electrolytes were prepared in the same manner as described inExample 1, except that the concentration of acid and the acid treatmenttime were varied for each of the first, second, and third acidtreatments as shown in Table 1 below.

TABLE 1 Concentration of Acid (M) Acid treatment time (min) Example 2-10.5 First acid treatment: 10 min Second acid treatment: 10 min Thirdacid treatment: 10 min Example 2-2 2 First acid treatment: 10 min Secondacid treatment: 10 min Third acid treatment: 10 min Example 2-3 4 Firstacid treatment: 10 min Second acid treatment: 10 min Third acidtreatment: 10 min Comparative 5 First acid treatment: 10 min Example 1-1Second acid treatment: 10 min Third acid treatment: 10 min Comparative 0.1M First acid treatment: 10 min Example 1-2 Second acid treatment: 10min Third acid treatment: 10 min Comparative 0.01M First acid treatment:10 min Example 1-3 Second acid treatment: 10 min Third acid treatment:10 min Comparative 5M HCl 30 min Example 1-4

Example 3: Preparation of Solid Electrolyte

Solid electrolytes were prepared in the same manner as described inExample 1, except that the concentration of acid and acid treatment timeused in first to third acid treatments were varied as shown in Table 2below. When the acid treatment is performed while gradually reducing theconcentration of the acid, as shown in Table 2, particle connections inthe porous layer are well maintained.

TABLE 2 First acid treatment Second acid treatment Third acid treatmentAcid Acid Acid Concentration Acid Treatment Concentration Acid TreatmentConcentration Acid Treatment (M) Time (min) (M) Time (min) (M) Time(min) Example 4 10 2 10 1 10 3-1 Example 2 10 1 10 0.5 10 3-2 Example 110 0.5 10 0.1 10 3-3

Example 5: Preparation of Solid Electrolyte

A solid electrolyte was prepared in the same manner as in Example 2,except that the acid treatment was performed on both surfaces of theLLZO film.

The acid treatment performed on both surfaces of the LLZO film includesimmersing the LLZO film in a 1 M HCl aqueous solution for about 10minutes, cleaning the film using ethanol, and drying the resultant.

Examples 6 and 7: Preparation of Solid Electrolyte

Solid electrolytes were prepared in the same manner as in Example 1,except that the LLZO films of Preparation Example 2 and of PreparationExample 3 were respectively used instead of the LLZO film of PreparationExample 1.

Example 7a: Preparation of Solid Electrolyte

A solid electrolyte was prepared in the same manner as in Example 1,except that the LLZO film of Preparation Example 5 was used instead ofthe LLZO film of Preparation Example 1.

Comparative Example 2: Preparation of Solid Electrolyte

A solid electrolyte was prepared in the same manner as in Example 1,except that the LLZO film prepared according to Comparative PreparationExample 1 was used instead of the LLZO film prepared according toPreparation Example 1.

Example 8: Preparation of Lithium Secondary Battery

First, a positive electrode was prepared according to the followingmethod.

LiNi_(0.33)Co_(0.33)Mn_(0.33)O₂(NCM), polyvinylidene fluoride (PVDF),Super P as a conductive agent, and N-methylpyrollidone were mixed toobtain a positive active material layer forming composition. In thepositive active material layer forming composition, a weight ratio ofNCM to Super P to PVDF was 97:1.5:1.5. An amount of N-methylpyrollidonewas about 137 grams (g) when an amount of NCM was 97 g.

The positive active material layer forming composition was coated on anAl foil (thickness: about 15 μm) and dried at 25° C., and the driedresultant was further dried in a vacuum at about 110° C. to prepare apositive electrode.

A lithium metal negative electrode (thickness: about 20 μm) was disposedon a current collector (Cu foil) and the lithium metal negativeelectrode and a solid electrolyte were bonded by applying a pressure ofabout 250 MPa by using a cold isostatic pressing (CIP) method.

The positive electrode prepared as described above was placed in astainless battery case and a positive electrolyte was added thereto.Then, the positive electrode was disposed on a surface of the solidelectrolyte prepared according to Example 1, the other surface of whichis mounted with the lithium metal negative electrode (thickness: 20 μm)to prepare a lithium secondary battery (refer to FIG. 3).

The positive electrolyte was a 2 M LiFSI ionic liquid prepared by mixingLiFSI, as a lithium salt, and N-methyl-N-propyl pyrrolidiniumbis(fluorosulfonyl) imide (Pyr13FSI, PYR₁₃+ cation structure) having thestructure shown below), as an ionic liquid.

Example 9 to 11: Preparation of Lithium Secondary Battery

Lithium secondary batteries were prepared in the same manner as inExample 8, except that the solid electrolytes prepared according toExamples 6, 7, and 7a were used instead of the solid electrolyte ofExample 1.

Comparative Example 3: Preparation of Lithium Secondary Battery

A lithium secondary battery was prepared in the same manner as inExample 8, except that the LLZO film prepared according to PreparationExample 1 was used instead of the solid electrolyte of Example 1.

Comparative Example 4: Preparation of Lithium Secondary Battery

A lithium secondary battery was prepared in the same manner as inExample 8, except that the solid electrolyte was prepared according toComparative Preparation Example 1-1, prepared as described withreference to “Garnet solid-state electrolyte preparation,” EnergyEnviron. Sci., 2017, 1568-1575, page 1573, incorporated herein byreference in its entirety, was used instead of the solid electrolyte ofExample 1.

Comparative Example 5: Preparation of Lithium Secondary Battery

A lithium secondary battery was prepared in the same manner as inExample 8, except that the solid electrolyte of Comparative Example 2was used instead of the solid electrolyte of Example 1.

Comparative Examples 6 and 7: Preparation of Lithium Secondary Battery

Lithium secondary batteries were prepared in the same manner as inExample 8, except that the solid electrolytes prepared according toComparative Preparation Examples 1-2 and 1-4 were respectively usedinstead of the solid electrolyte of Example 1.

Comparative Example 8

A lithium secondary battery was prepared in the same manner as inExample 8, except that poly(vinylidene fluoride (PVDF) was used as aseparator instead of the LLZO solid electrolyte, and Li metal was bondedthereto without performing the CIP method.

Evaluation Example 1: Scanning Electron Microscopy (SEM)

A cross-sections of the solid electrolytes prepared according to Example1 and Comparative Preparation Example 1-1 were observed by using ascanning electron microscope (SEM). An SU8030 available from Hitachi,Ltd., was used as the SEM. SEM images of the solid electrolyte ofExample 1 are shown in FIGS. 4A and 4B. FIG. 4B is an enlarged view ofthe oval area in FIG. 4A outlined by dashed lines. In addition, an SEMimage of the LLZO film prepared according to Comparative PreparationExample 1-1 is shown in FIG. 4C.

Referring to FIG. 4A, a second porous layer 42 having a thickness ofabout 10 μm and a pore size of 5 μm or less is formed on an LLZO film41, and a first porous layer 43 having a thickness of about 20 μm and apore size greater than 20 μm is formed thereon. A total thickness of thesecond porous layer 42 and the first porous layer 43 is about 30 μm. Inaddition, referring to FIG. 4B, small grains are selectively dissolvedby acid treatment with a strong acid to form pores and surfaces of theremaining grains are partially dissolved such that the remaining grainshave a polyhedral shape deviating from the circular shape.

Referring to FIG. 4C, it may be confirmed that the same pore size isdistributed over the entire porous layer with no open pores on thesurface when the porous layer is formed by screen printing.

Pore size distribution and mechanical strength (Ring on Ring test) ofthe solid electrolytes prepared according to Examples 6 and 7 andComparative Example 1-1 were measured. The pore size distribution wasmeasured by using Autopore IV 9520 available from MicromeriticsInstrument Corp. and the mechanical strength was evaluated according tothe Ring on Ring test, by using a MTS 10D Load Frame, with 0.75 inchdiameter support ring, and 0.25 inch diameter load ring available fromSintech corporation.

Pore size distributions of the solid electrolytes prepared according toExamples 6 and 7 is shown in FIG. 5A, and Pore size distributions of thesolid electrolytes prepared according to Comparative Example 2 is shownin FIG. 5B.

Evaluation results for the mechanical strength of the solid electrolytesprepared according to Example 6 and Comparative Example 2 are shown inTable 3.

TABLE 3 RoR (MPa) Example 6 139 Comparative Example 2 126

As illustrated in FIGS. 5A and 5B, it was confirmed that the solidelectrolyte prepared according to Example 6 had a wide pore sizedistribution (200 μm to 0.5 μm) by surface treatment using a 1 M HClaqueous solution compared with the solid electrolyte prepared accordingto Comparative Example 2. The solid electrolyte prepared according toExample 6 had a structure in which the pore size gradually decreasesfrom the surface of the solid electrolyte to the inside thereof. Inaddition, the solid electrolyte of Example 6 had a similar level ofmechanical strength to that of the solid electrolyte of ComparativeExample 2 that does not include a porous layer. In addition, themechanical strength of the solid electrolyte of Example 7 was similar tothat of the solid electrolyte of Example 6.

Evaluation Example 2: XRD Characteristics

X-ray diffraction (XRD) analysis was performed on the LLZO film ofPreparation Example 1 and the solid electrolyte of Example 1. XRDanalysis was performed by using a D8 Advance available from Bruker.

XRD analysis results are shown in FIGS. 6A and 6B. FIG. 6B is anenlarged view of the area in FIG. 6A marked with a circle. In addition,XRD analysis was performed on the LLZO film of Preparation Example 1 andthe solid electrolyte of Example 1 and spacer group, lattice constant a,and average grain size thereof were measured and shown in Table 4. FIG.6A, and FIG. 6B.

TABLE 4 Space group a (Å) Average grain size (nm) Preparation Ia-3d(cubic) 12.958 48.1 Example 1 Example 1 1a-3d (cubic) 12.968 279.2

Referring to FIGS. 6A and 6B and Table 4, it was confirmed thatcrystallographic characteristics of the solid electrolyte preparedaccording to Example 1 have been changed since lithium ions aresubstituted with protons in comparison with the LLZO film preparedaccording to Preparation Example 1. As a result of XRD analysis, theacid-treated solid electrolyte had an increased crystallinity and anincreased lattice constant due to proton substitution and an increasedaverage grain size due to a decrease in the number of small grains. Thiswas consistent with analysis results of SEM images.

Evaluation Example 3: Impedance Characteristics

Impedance of the lithium secondary batteries of Example 8 andComparative Example 3 was measured. Impedance was measured by using animpedance analyzer (Solartron 1260A Impedance/Gain-Phase Analyzer)according to a 2-probe method at 60° C. in a frequency range of about10⁶ megahertz (MHz) to about 0.1 MHz while applying an AC voltage biasof 10 millivolts (mV) in an open-circuit state before lifespanevaluation.

Impedance results are shown in FIGS. 7A and 7B. FIGS. 7A and 7B aregraphs illustrating the impedance characteristics of lithium secondarybatteries prepared according to Example 8 and Comparative Example 3.

Referring to FIGS. 7A and 7B, an overall interfacial resistance of thelithium secondary batteries of Example 8 was reduced compared with thatof the lithium secondary batteries of Comparative Example 3. Theinterfacial resistance decreased since the surface area increased due toformation of the porous layer on the solid electrolyte and surfaceimpurities such as lithium carbonate (Li₂CO₃) are removed due to anincrease in interfacial active areas between the solid electrolyte andthe lithium metal electrode.

Evaluation Example 4: Charge/Discharge Characteristics

Charge/discharge characteristics of the lithium secondary batteriesprepared according to Example 8 and Comparative Examples 3 and 4 wereevaluated by a galvanostatic method at a current density of 0.64 mA/cm²under the following conditions.

Each lithium secondary battery was charged in a constant current mode at60° C. at a current of 0.3 mA/cm² until a voltage reached 4.0 V (vs. Li)and cut-off at a current of 0.15 mA/cm² while maintaining the voltage of4.0 V in a constant voltage mode. Then, the lithium secondary batterywas discharged at a constant current of 0.3 mA/cm² until the voltagereached 2.8 V (vs. Li) (1^(st) cycle, formation cycle).

Charge and discharge of the lithium secondary batteries were repeated1800 times at 60° C., and at a current of 0.5 mA/cm² and 0.64 mA/cm²,respectively.

Evaluation results of charge/discharge characteristics of the lithiumsecondary batteries prepared according to Example 8 and ComparativeExamples 3 and 4 are shown in FIGS. 8A to 8C, respectively.

As shown in FIGS. 8B and 8C, in the lithium secondary batteriesaccording to Comparative Examples 3 and 4, voltage noise due to shortageof Li was observed during the 3^(rd) and 4^(th) charging operations withcharge and discharge of the batteries in a constant current(galvanostatic method) at a current density of 0.64 mA/cm².

On the contrary, in the lithium secondary battery prepared according toExample 8, high capacity retention ratios were observed without a shortcircuit during charge and discharge at least up to the 10^(th) cycle.Without being limited by any theory, it is believed that this is becausethe surface porous layer formed by acid treatment forms a stableinterface with lithium metal, and thus Li plating and concentration ofstress are prevented at grain boundaries, thereby inhibiting penetrationof Li through the solid electrolyte.

Evaluation Example 5: Charge/Discharge Characteristics 1) Example 8 andComparative Example 8

To confirm lifespan characteristics of the lithium secondary batteriesprepared according to Example 8 and Comparative Example 8 at a highcurrent density, each cell having a capacity of 2 mAh/cm² was subjectedto a charge and discharge test at an increased current density of 3mA/cm². Charge and discharge test results of the lithium secondarybattery of Example 8 are shown in FIGS. 9A and 9B and charge anddischarge test results of the lithium secondary battery of ComparativeExample 8 is shown in FIG. 9C.

FIG. 9A is a graph illustrating electrode potential of lithium secondarybattery prepared according to Example 8; and FIG. 9B is a graphillustrating electrode potential of a lithium secondary battery preparedaccording to Example 8 with respect to capacity. FIG. 9C is a graphillustrating electrode potential of lithium secondary battery preparedaccording to Comparative Example 8 with respect to the number of cycles:

Each lithium secondary battery was charged in a constant current mode at60° C. at a current of 0.3 mA/cm² until a voltage reached 4.0 V (vs. Li)and cut-off at a current of 0.15 mA/cm² while maintaining the voltage of4.0 V in a constant voltage mode. Then, the lithium secondary batterywas discharged at a constant current of 0.3 mA/cm² until the voltagereached 2.8 V (vs. Li) (1^(st) cycle, formation cycle).

Charge and discharge of the lithium secondary batteries were repeated(1800 times) at 60° C. at a current of 0.75 and 0.8 mA/cm²,respectively.

As illustrated in FIGS. 9A and 9B, it may be confirmed that the lithiumsecondary battery prepared according to Example 8 may operate over 1000cycles without short circuits and rapid decrease in capacity. It mayalso be confirmed that the lithium secondary battery of Example 8 hadexcellent lifespan characteristics by using the solid electrolyte havingthe porous layer formed by acid treatment. On the contrary, Asillustrated in FIG. 9C, lifespan characteristics of the lithiumsecondary battery of Comparative Example 8 using an ionic liquidelectrolyte and a separator deteriorate due to continuous shortage of Licaused by side reactions between the liquid electrolyte and lithiummetal. As illustrated in FIG. 9C, the lithium secondary battery ofComparative Example 8 had a capacity retention ratio of 80% or lesswithin 70 cycles.

2) Examples 8 and 11 and Comparative Examples 6 and 7

To confirm electrode potential variation of the lithium secondarybatteries prepared according to Examples 8 and 11 and ComparativeExamples 6 and 7 with respect to capacity, each cell having a capacityof 2 mAh/cm² was subjected to a charging and discharging test at anincreased current density of 3 mA/cm². Charge and discharge test resultsof the lithium secondary batteries of Examples 8 and 11 are shown inFIGS. 13A and 13B and charging and discharging test results of thelithium secondary batteries of Comparative Examples 6 and 7 are shown inFIGS. 14A and 14B.

Each lithium secondary batteries was charged in a constant current modeat 60° C. at a current of 0.3 mA/cm² until a voltage reached 4.0 V (vs.Li) and cut-off at a current of 0.15 mA/cm² while maintaining thevoltage of 4.0 V in a constant voltage mode. Then, the lithium secondarybatteries were discharged at a constant current of 0.3 mA/cm² until thevoltage reached 2.8 V (vs. Li) (1^(st) cycle, formation cycle).

Charging and discharging of the lithium secondary batteries wererepeated 1800 times at 60° C. at a current of 0.3 mA/cm², 0.5 mA/cm²,and 1.6 mA/cm², respectively.

Based on the results, it may be confirmed that the lithium secondarybatteries prepared according to Examples 8 and 11 have better electrodepotential characteristics than the lithium secondary batteries preparedaccording to Comparative Examples 6 and 7, due to the use of the solidelectrolytes having the porous layers formed by acid treatment. Withoutbeing limited by any theory, it is understood that excellent lifespancharacteristics of the lithium secondary batteries of Examples 8 and 11may be obtained since the effect of the porous layer having a thicknessof about 30 μm on inhibiting lithium penetration is improved in thelithium secondary battery of Example 8 and since pores are efficientlyformed by maintaining connection states of acid-treated particles byenlarging grain boundaries in the lithium secondary battery of Example11.

Evaluation Example 6: X-ray Photoelectron Spectroscopy (XPS) Analysis

The LLZO film prepared according to Preparation Example 1 and the solidelectrolyte prepared according to Example 1 were subjected to XPSanalysis. The XPS analysis was performed using a Quantum 2000 (availablefrom Physical Electronics, Inc., acceleration voltage: 0.5kiloelectronvolts (keV) to 15 keV, 300 watts (W), energy resolution:about 1.0 eV, and Sputter rate: 0.1 nanometer per minute (nm/min)).

XPS analysis results are shown in FIGS. 10A to 10F. FIGS. 10A to 10C aregraphs illustrating Li 1s, C 1s, and O 1s XPS analysis results of anLLZO Film prepared according to Preparation Example 1, respectively, andFIGS. 10D to 10F are graphs illustrating Li 1s, C 1s, and O 1s XPSanalysis results of a solid electrolyte prepared according to Example 1,respectively.

Referring to the results of FIGS. 10A to 10F, it may be confirmed thatthe Li₂CO₃ present in the surface and a sub-surface (grain boundary) mayalso be decomposed and removed by acid-treating the surface of the solidelectrolyte. As a result of XPS depth analysis performed on the surfaceof the solid electrolyte by Ar sputtering, it may be confirmed that theamount of Li₂CO₃ was considerably reduced in the acid-treated solidelectrolyte up to the sub-surface. Without being limited by theory,since Li₂CO₃ is an inactive material having a low electronicconductivity and a low ionic conductivity, rate characteristics of thelithium secondary battery deteriorate due to an increase in resistanceof charge-transfer reaction in interfaces between the solid electrolyteand the positive electrode and the negative electrode in the presence ofLi₂CO₃. In addition, Li₂CO₃ may form lithium carbide (Li_(x)C: 0<x<1) byreduction decomposition reactions during charging/discharging to grow Lidendrite, thereby causing short circuits. Thus, rate characteristics andlifespan characteristics of the lithium secondary battery may beimproved by using the solid electrolyte from which Li₂CO₃ was removed byacid treatment.

Evaluation Example 7: Aging Test

After exposing the solid electrolytes used in the lithium secondarybatteries according to Example 8 and Comparative Example 3 to air forabout 18 hours, an Li-LLZO-Li symmetric cell was prepared and impedancethereof was measured. Resistance was measured using an impedanceanalyzer (Solartron 1260A Impedance/Gain-Phase Analyzer) according to a2-probe method at 60° C. in a frequency range of about 10⁶ MHz to about0.1 MHz while applying an AC voltage bias of 10 mV thereto.

Impedance results of the lithium secondary batteries of ComparativeExample 3 and Example 8 are shown in FIGS. 11A and 11B, respectively.

Referring to the results, impedance of the solid electrolyte of Example8 decreased after being exposed to oxygen in comparison with ComparativeExample 3. Thus, it was confirmed that stability in air was improvedsince formation of lithium carbonate is inhibited on the surface of thesolid electrolyte due to substitution of lithium ions with protons.

Evaluation Example 8: Laser Induced Breakdown Spectroscopy (LIBS)

The solid electrolyte prepared according to Example 1 and the LLZO filmprepared according to Preparation Example 1 were analyzed by LIBS. TheLIBS was performed using a J200 available from Applied Spectra andanalysis results are shown in FIGS. 12A and 12B.

Referring to the results, it was confirmed that the surface of the solidelectrolyte of Example 1 was partially substituted with protons by acidtreatment and the amount of the protons decreased in an inward directiontoward the inside of the bulk material.

Since the solid electrolyte according to an embodiment has an increasedcontact area with an electrode, interfacial resistance is reduced. Inaddition, since lithium penetration at grain boundaries of the solidelectrolyte is inhibited, short circuits may be reduced. By using thesolid electrolyte, secondary batteries having excellent lifespancharacteristics may be prepared.

It should be understood that embodiments described herein should beconsidered in a descriptive sense only and not for purposes oflimitation. Descriptions of features or aspects within each embodimentshould be considered as available for other similar features or aspectsin other embodiments.

While one or more embodiments have been described with reference to thefigures, it will be understood by those of ordinary skill in the artthat various changes in form and details may be made therein withoutdeparting from the spirit and scope as defined by the following claims.

What is claimed is:
 1. A solid electrolyte comprising: an inorganiclithium ion conductive film; and a porous layer on a surface of theinorganic lithium ion conductive film, wherein the porous layercomprises a first porous layer and a second porous layer, and the secondporous layer is disposed between the inorganic lithium ion conductivefilm and the first porous layer, and wherein the first porous layer hasa pore size which is greater than a pore size of the second porouslayer.
 2. The solid electrolyte of claim 1, wherein a porosity of thefirst porous layer is greater than a porosity of the second porouslayer.
 3. The solid electrolyte of claim 1, wherein a pore size of theporous layer increases in a thickness direction of the solidelectrolyte.
 4. The solid electrolyte of claim 3, wherein the pore sizeof the porous layer increases in a direction from the inorganic lithiumion conductive film to an outer surface of the porous layer.
 5. Thesolid electrolyte of claim 1, wherein the porous layer has a totalthickness and density to be impermeable to a liquid.
 6. The solidelectrolyte of claim 1, wherein the porous layer of the solidelectrolyte has a thickness of about 5% to about 95% of the totalthickness of the solid electrolyte.
 7. The solid electrolyte of claim 1,wherein an average pore size of a pore in the porous layer is from about0.1 micrometer to about 1,000 micrometers.
 8. The solid electrolyte ofclaim 1, wherein a pore in the first porous layer has an average poresize of about 10 micrometers to about 500 micrometers, and a pore in thesecond porous layer has an average pore size of about 0.1 micrometer toabout 10 micrometers.
 9. The solid electrolyte of claim 1, wherein atleast a portion of the solid electrolyte comprises an inorganic lithiumion conductor comprising lithium, and a portion of the lithiumsubstituted by a proton.
 10. The solid electrolyte of claim 9, whereinan amount of protons in the porous layer is from about 0.01 mole percentto about 50 mole percent based on the total number of protons andlithium ions.
 11. The solid electrolyte of claim 1, wherein the firstporous layer comprises a first inorganic lithium ion conductorsubstituted with about 2% to about 100% protons, and the second porouslayer solid electrolyte comprises a second inorganic lithium ionconductor substituted with about 0.01% to about 20% protons, based onthe total number of protons and lithium ions.
 12. The solid electrolyteof claim 1, wherein the porous layer is a product obtained by:acid-treating an inorganic lithium ion conductive film with an acidhaving a concentration of greater than or equal to about 0.1 molar andless than or equal to about 5 molar.
 13. The solid electrolyte of claim11, wherein the concentration of the acid is from about 0.5 molar toabout 4.5 molar.
 14. The solid electrolyte of claim 1, wherein theporous layer is a product obtained by: forming a multilayer filmcomprising two or more layers on an inorganic lithium ion conductivefilm; and heat-treating the multilayer film, wherein the forming of themultilayer film comprises coating a first composition comprising a poreformer on a surface of the inorganic lithium ion conductive film andcoating a second composition comprising a pore former on the firstcomposition, wherein the amount of the pore former in the firstcomposition is different from the amount of the pore former in thesecond composition.
 15. The solid electrolyte of claim 1, wherein alattice constant of the porous layer as measured by X-ray diffraction isgreater than a lattice constant of the remaining area of the solidelectrolyte excluding the porous layer by about 0.005 angstrom to about0.1 angstrom, and an average grain size of the porous layer is greaterthan an average grain size of the remaining area of the solidelectrolyte excluding the porous layer by twice or more.
 16. The solidelectrolyte of claim 1, wherein the inorganic lithium ion conductivefilm comprises at least one of a garnet compound, an argyroditecompound, a lithium super-ion-conductor compound, a Li nitride, a Lihydride, a perovskite, or a Li halide.
 17. The solid electrolyte ofclaim 1, wherein the inorganic lithium ion conductive film comprises atleast one of Li_(3+x)La₃M₂O₁₂ wherein 0≤x≤5 and M is W, Ta, Te, Nb, Zror a combination thereof, Li_(3+x)La₃M₂O₁₂ wherein 0≤x≤5 and M is W, Ta,Te, Nb, Zr or a combination thereof,Li_(1+x+y)Al_(x)Ti_(2-x)Si_(y)P_(3-y)O₁₂ wherein 0<x<2 and 0≤y<3,BaTiO₃, Pb(Zr_(a)Ti_(1-a))O₃ wherein 0≤a≤1,Pb_(1-x)La_(x)Zr_(1-y)Ti_(y)O₃ wherein 0≤x<1 and 0≤y<1,Pb(Mg_(1/3)Nb_(2/3))O₃—PbTiO₃, Li₃PO₄, Li_(x)Ti_(y)(PO₄)₃ wherein 0<x<2and 0<y<3, Li_(x)Al_(y)Ti_(z)(PO₄)₃ wherein 0<x<2, 0<y<1, and 0<z<3,Li_(1+x+y)(Al,Ga)_(x)(Ti,Ge)_(2-x)Si_(y)P_(3-y)O₁₂ wherein 0≤x≤1 and0≤y≤1, Li_(x)La_(y)TiO₃, wherein 0<x<2, and 0<y<3,Li_(x)Ge_(y)P_(z)S_(w) wherein 0<x<4, 0<y<1, 0<z<1, and 0<w<5,Li_(x)N_(y) wherein 0<x<4, and 0<y<2, Li_(x)Si_(y)S_(z) wherein 0≤x<3,0<y<2, and 0<z<4, Li_(x)P_(y)S_(z) wherein 0≤x<3, 0<y<3, and 0<z<7,Li_(3x)La_(2/3-x)TiO₃ wherein 0≤x≤⅙, Li_(1+y)Al_(y)Ti_(2-y)(PO₄)₃wherein 0≤y≤1, Li_(1+z)Al_(z)Ge_(2-z)(PO₄)₃ wherein 0≤z≤1, Li₂O, LiF,LiOH, Li₂CO₃, LiAlO₂, a Li₂O—Al₂O₃—SiO₂—P₂O₅—TiO₂—GeO₂ ceramic,Li₇La₃Zr₂O₁₂, Li₁₀GeP₂S₁₂, Li_(3.25)Ge_(0.25)P_(0.75)S₄, Li₃PS₄,Li₆PS₅Br, Li₆PS₅Cl, Li₇PS₅, Li₆PS₅I, Li_(1.3)Al_(0.3)Ti_(1.7)(PO₄)₃,LiTi₂(PO₄)₃, LiGe₂(PO₄)₃, LiHf₂(PO₄)₃, LiZr₂(PO₄)₃, Li₂NH, Li₃(NH₂)₂I,LiBH₄, LiAlH₄, LiNH₂, Li_(0.34)La_(0.51)TiO_(2.94), LiSr₂Ti₂NbO₉,Li_(0.06)La_(0.66)Ti_(0.93)Al_(0.03)O₃, Li_(0.34)Nd_(0.55)TiO₃,Li₂CdCl₄, Li₂CdCl₄, Li₂MgCl₄, Li₂ZnI₄, Li₂CdI₄,Li_(4.9)Ga_(0.5+δ)La₃Zr_(1.7)W_(0.3)O₁₂ wherein 0≤δ<1.6,Li_(4.9)Ga_(0.5+δ)La₃Zr_(1.7)W_(0.3)O₂ wherein 1.7≤δ≤2.5, orLi_(5.39)Ga_(0.5+δ)La₃Zr_(1.7)W_(0.3)O₁₂ wherein 0≤δ<1.11.
 18. The solidelectrolyte of claim 1, wherein the inorganic lithium ion conductivefilm comprises at least one of a compound of Formula 1 and a compound ofFormula 1a:Li_(7-x)M¹ _(x)La_(3-a)M² _(a)Zr_(2-b)M³ _(b)O₁₂, and  Formula 1Li_(7-x)La_(3-a)M² _(a)Zr_(2-b)M³ _(b)O₁₂,  Formula 1a wherein, inFormula 1, M¹ comprises at least one of gallium or aluminum, in Formulas1 and 1a, M² comprises at least one of calcium, strontium, cesium, orbarium, M³ comprises at least one of aluminum, tungsten, niobium, ortantalum, and 0≤x<3, 0≤a≤3, and 0≤b<2.
 19. The solid electrolyte ofclaim 1, wherein the inorganic lithium ion conductive film comprises atleast one of Li₇La₃Zr₂O₁₂, Li_(6.4)La₃Zr_(1.7)W_(0.3)O₁₂,Li_(6.5)La₃Zr_(1.5)Ta_(0.3)O₁₂, Li₇La₃Zr_(1.7)W_(0.3)O₁₂,Li_(4.9)La_(2.5)Ca_(0.5)Zr_(1.7)Nb_(0.3)O₁₂,Li_(4.9)Ga_(2.1)La₃Zr_(1.7)W_(0.3)O₁₂, Li₇La₃Zr_(1.5)W_(0.5)O₁₂,Li₇La_(2.75)Ca_(0.25)Zr_(1.75)Nb_(0.25)O₁₂, Li₇La₃Zr_(1.5)Nb_(0.5)O₁₂,Li₇La₃Zr_(1.5)Ta_(0.5)O₁₂, Li_(6.272)La₃Zr_(1.7)W_(0.3)O₁₂, orLi_(5.39)Ga_(1.61)La₃Zr_(1.7)W_(0.3)O₁₂.
 20. The solid electrolyte ofclaim 1, wherein a surface of the solid electrolyte comprisesLi_(7-x)H_(x)La₃Zr_(2-y)M_(y)O₁₂ wherein 0.1≤x≤7, 0≤y≤2, and M is atleast one of W, Ta, Te, or Nb, and the interior of the solid electrolytecomprises Li_(7-x)H_(x)La₃Zr_(2-y)M_(y)O₁₂ wherein 0≤x≤6.5, 0≤y≤2, and Mis at least one of W, Ta, Te, or Nb).
 21. The solid electrolyte of claim1, wherein the first porous layer defines a surface of the solidelectrolyte and has a porosity of about 5% to about 80%, and the secondporous layer has a porosity of about 1% to about 50%.
 22. The solidelectrolyte of claim 1, wherein the first porous layer comprises alithium ion conductor substituted with Li_(7-x)H_(x)La₃Zr₂O₁₂ wherein0.1≤x≤7) and the second porous layer comprises Li_(7-x)H_(x)La₃Zr₂O₁₂wherein 0≤x≤6.5.
 23. The solid electrolyte of claim 1, wherein the firstporous layer comprises a lithium ion conductor substituted withLi_(6.75-x)H_(x)La_(2.9)Ga_(0.1)Nb_(0.25)Zr_(1.75)O₁₂ wherein0.1≤x≤6.75, and the second porous layer comprises Li_(7-x)H_(x)La₃Zr₂O₁₂wherein 0≤x≤6.5.
 24. The solid electrolyte of claim 1, wherein theporous layer has a porosity of about 5% to about 60%, based on a totalporosity of the porous layer.
 25. The solid electrolyte of claim 1,wherein inorganic lithium ion conductive film comprises an inorganiclithium ion conductor having a particle structure or a columnarstructure.
 26. The solid electrolyte of claim 25, wherein a grain of theinorganic lithium ion conductor has a polyhedral shape.
 27. A secondarybattery comprising: a positive electrode, a negative electrode, and thesolid electrolyte according to claim 1 interposed between the positiveelectrode and the negative electrode.
 28. The secondary battery of claim27, further comprising at least one of a liquid electrolyte, a polymerelectrolyte, a lithium salt, an ionic liquid, or a polymer ionic liquid.29. The secondary battery of claim 27, further comprising a solidsulfide electrolyte between the negative electrode and the solidelectrolyte.
 30. The secondary battery of claim 29, wherein the solidsulfide electrolyte comprises Li₂S—P₂S₅—LiX wherein X is at least one ofF, Cl, Br, or I.
 31. The secondary battery of claim 27, wherein thenegative electrode comprises at least one of a lithium metal or alithium metal alloy.
 32. The secondary battery of claim 27, wherein asurface of the solid electrolyte in contact with the negative electrodehas a greater pore size than a surface of the solid electrolyte not incontact with the positive electrode, and the pore size in the solidelectrolyte decreases in a direction extending away from the negativeelectrode in the solid electrolyte.
 33. The secondary battery of claim27, wherein the secondary battery is configured to operate greater than1000 cycles at current density of 3 milliamperes per square centimeter.34. A method of preparing the solid electrolyte, the method comprising:a first acid treatment comprising acid-treating an inorganic lithium ionconductive film with an acid having a concentration of greater than orequal to about 0.1 molar and less than or equal to about 5 molar toprovide a first acid-treated product; and a first cleaning comprisingcleaning the first acid-treated product to provide a cleaned firstacid-treated product to prepare the solid electrolyte.
 35. The method of34, further comprising preparing the inorganic lithium ion conductivefilm by combining an inorganic lithium ion conductor and a lithiumcompound to prepare a mixture, and heat-treating the mixture.
 36. Themethod of claim 35, wherein the lithium compound comprises at least oneof lithium carbonate, lithium hydroxide, or lithium oxide, and an amountof the lithium compound is from about 0.1 parts by weight to about 10parts by weight, based on 100 parts by weight of a total weight of theinorganic lithium ion conductor and the lithium compound.
 37. The methodof claim 34, further comprising a second acid treatment comprisingacid-treating the cleaned first acid-treated product with an acid havinga concentration of greater than or equal to about 0.1 molar and lessthan or equal to about 5 molar to provide a second acid-treated product,and a second cleaning comprising cleaning the second acid-treatedproduct.
 38. The method of claim 37, wherein a concentration of the acidused in the second acid treatment is less than a concentration of theacid used in the first acid treatment.
 39. A method of preparing a solidelectrolyte, the method comprising: forming a multilayer film on asurface of an inorganic lithium ion conductive film, the formingcomprising: coating a first composition comprising a pore former and alithium ion conductor on a surface of the inorganic lithium ionconductive film, and drying the first composition, and coating a secondcomposition comprising a pore former and a lithium ion conductor on thedried first composition, and drying the second composition to form themultilayer film; and heat-treating the multilayer film to prepare thesolid electrolyte.
 40. The method of claim 39, wherein the pore formercomprises at least one of dibutyl phthalate, dimethyl phthalate, diethylphthalate, dioctyl sebacate, or dioctyl adipate.